Enhanced dorsal horn stimulation using multiple electrical fields

ABSTRACT

A method of providing therapy to a patient having a medical condition comprises delivering electrical stimulation energy to the spinal cord of the patient in accordance with a stimulation program that preferentially stimulates dorsal horn neuronal elements over dorsal column neuronal elements in the spinal cord. The delivered electrical stimulation energy generates a plurality of electrical fields having different orientations that stimulate the dorsal horn neuronal elements.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.14/805,741, filed Jul. 22, 2015, which claims the benefit of priorityunder 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No.62/028,643, filed on Jul. 24, 2014, which is herein incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to implantable medical systems, and moreparticularly, to systems and methods for stimulating tissue.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. For example, Spinal Cord Stimulation(SCS) techniques, which directly stimulate the spinal cord tissue of thepatient, have long been accepted as a therapeutic modality for thetreatment of chronic neuropathic pain syndromes, and the application ofspinal cord stimulation has expanded to include additional applications,such as angina pectoralis, peripheral vascular disease, andincontinence, among others. Spinal cord stimulation is also a promisingoption for patients suffering from motor disorders, such as Parkinson'sDisease, Dystonia and essential tremor.

SCS systems typically include one or more electrode carrying stimulationleads, which are implanted at the desired stimulation site, and aneurostimulator (e.g., an implantable pulse generator (IPG)) implantedremotely from the stimulation site, but coupled either directly to theneurostimulation lead(s) or indirectly to the neurostimulation lead(s)via a lead extension.

Electrical stimulation energy may be delivered from the IPG to theelectrodes in the form of an electrical pulsed waveform. Thus,electrical pulses can be delivered from the IPG to the neurostimulationleads to stimulate the spinal cord tissue and provide the desiredefficacious therapy to the patient. The configuration of electrodes usedto deliver electrical pulses to the targeted spinal cord tissueconstitutes an electrode configuration, with the electrodes capable ofbeing selectively programmed to act as anodes (positive), cathodes(negative), or left off (zero). In other words, an electrodeconfiguration represents the polarity being positive, negative, or zero.Other parameters that may be controlled or varied include the amplitude,pulse width, and rate (or frequency) of the electrical pulses providedthrough the electrode array. Each electrode configuration, along withthe electrical pulse parameters, can be referred to as a “stimulationparameter set.”

The SCS system may further comprise a handheld patient programmer in theform of a remote control (RC) to remotely instruct the IPG to generateelectrical stimulation pulses in accordance with selected stimulationparameters. Typically, the stimulation parameters programmed into theIPG can be adjusted by manipulating controls on the RC to modify theelectrical stimulation provided by the IPG system to the patient. Thus,in accordance with the stimulation parameters programmed by the RC,electrical pulses can be delivered from the IPG to the stimulationelectrode(s) to stimulate or activate a volume of tissue in accordancewith a set of stimulation parameters and provide the desired efficacioustherapy to the patient. The best stimulus parameter set will typicallybe one that delivers stimulation energy to the volume of tissue thatmust be stimulated in order to provide the therapeutic benefit (e.g.,treatment of pain), while minimizing the volume of non-target tissuethat is stimulated.

However, the number of electrodes available combined with the ability togenerate a variety of complex electrical pulses, presents a hugeselection of stimulation parameter sets to the clinician or patient. Forexample, if the SCS system to be programmed has an array of sixteenelectrodes, millions of stimulation parameter sets may be available forprogramming into the SCS system. Today, SCS systems may have up tothirty-two electrodes, thereby exponentially increasing the number ofstimulation parameters sets available for programming.

To facilitate such selection, the clinician generally programs the IPGthrough a computerized programming system; for example, a clinician'sprogrammer (CP). The CP can be a self-contained hardware/softwaresystem, or can be defined predominantly by software running on astandard personal computer (PC). The CP may actively control thecharacteristics of the electrical stimulation generated by the IPG toallow the optimum stimulation parameters to be determined based onpatient feedback or other means and to subsequently program the IPG withthe optimum stimulation parameter sets.

For example, in order to achieve an effective result from conventionalSCS, the lead or leads must be placed in a location, such that theelectrical stimulation energy creates a sensation known as paresthesia,which can be characterized as an alternative sensation that replaces thepain signals sensed by the patient. The paresthesia induced by thestimulation and perceived by the patient should be located inapproximately the same place in the patient's body as the pain that isthe target of treatment. If a lead is not correctly positioned, it ispossible that the patient will receive little or no benefit from animplanted SCS system. Thus, correct lead placement can mean thedifference between effective and ineffective pain therapy. When leadsare implanted within the patient, the CP, in the context of an operatingroom (OR) mapping procedure, may be used to instruct the IPG to applyelectrical stimulation to test placement of the leads and/or electrodes,thereby assuring that the leads and/or electrodes are implanted ineffective locations within the patient.

Once the leads are correctly positioned, a fitting procedure, which maybe referred to as a navigation session, may be performed using the CP toprogram the RC, and if applicable the IPG, with a set of stimulationparameters that best addresses the painful site. Thus, the navigationsession may be used to pinpoint the VOA or areas correlating to thepain. Such programming ability is particularly advantageous fortargeting the tissue during implantation, or after implantation shouldthe leads gradually or unexpectedly move that would otherwise relocatethe stimulation energy away from the target site. By reprogramming theIPG (typically by independently varying the stimulation energy on theelectrodes), the VOA can often be moved back to the effective pain sitewithout having to re-operate on the patient in order to reposition thelead and its electrode array. When adjusting the VOA relative to thetissue, it is desirable to make small changes in the proportions ofcurrent, so that changes in the spatial recruitment of nerve fibers willbe perceived by the patient as being smooth and continuous and to haveincremental targeting capability.

Conventional SCS programming has as its therapeutic goal maximalstimulation (i.e., recruitment) of dorsal column (DC) nerve fibers thatrun in the white matter along the longitudinal axis of the spinal cordand minimal stimulation of other fibers that run perpendicular to thelongitudinal axis of the spinal cord (dorsal root (DR) nerve fibers,predominantly), as illustrated in FIG. 1. The white matter of the dorsalcolumn includes mostly large myelinated axons that form afferent fibers.Thus, conventionally, the large sensory afferents of the DC nerve fibershave been targeted for stimulation at an amplitude that provides painrelief.

While the full mechanisms are pain relief are not well understood, it isbelieved that the perception of pain signals is inhibited via the gatecontrol theory of pain, which suggests that enhanced activity ofinnocuous touch or pressure afferents via electrical stimulation createsinterneuronal activity within the dorsal horn (DH) of the spinal cordthat releases inhibitory neurotransmitters (Gamma-Aminobutyric Acid(GABA), glycine), which in turn, reduces the hypersensitivity of widedynamic range (WDR) sensory neurons to noxious afferent input of painsignals traveling from the dorsal root (DR) neural fibers that innervatethe pain region of the patient, as well as treating general WDR ectopy.Consequently, stimulation electrodes are typically implanted within thedorsal epidural space to provide stimulation to the DC nerve fibers.

As illustrated in FIG. 1, the DH can be characterized as central“butterfly” shaped central area of gray matter (neuronal cell bodies)substantially surrounded by an ellipse-shaped outer area of white matter(myelinated axons). The DH is the dorsal portion of the “butterfly”shaped central area of gray matter, which includes neuronal cellterminals, neuronal cell bodies, dendrites, and axons.

Activation of large sensory fibers also typically creates theparesthesia sensation that often accompanies SCS therapy. Althoughalternative or artifactual sensations, such as paresthesia, are usuallytolerated relative to the sensation of pain, patients sometimes reportthese sensations to be uncomfortable, and therefore, they can beconsidered an adverse side-effect to neuromodulation therapy in somecases.

It has been shown that the neuronal elements (e.g., neurons, dendrites,axons, cell bodies, and neuronal cell terminals) in the DH can bepreferentially stimulated over the DC neuronal elements by minimizingthe longitudinal gradient of an electrical field generated by aneurostimulation lead along the DC, thereby providing therapy in theform of pain relief without creating the sensation of paresthesia. Sucha technique is described in U.S. Provisional Patent Application Ser. No.61/911,728, entitled “Systems and Methods for Delivering Therapy to theDorsal Horn of a Patient,” which is expressly incorporated herein byreference.

This technique relies, at least partially on the natural phenomenon thatDH fibers and DC fibers have different responses to electricalstimulation. The strength of stimulation (i.e., depolarizing orhyperpolarizing) of the DC fibers and neurons is described by theso-called “activating function” ∂²V/∂x² which is proportional to thesecond-order spatial derivative of the voltage along the longitudinalaxis of the spine. This is partially because the large myelinated axonsin DC are primarily aligned longitudinally along the spine. On the otherhand, the likelihood of generating action potentials in DH fibers andneurons is described by the “activating function”∂V/∂x (otherwise knownas the electric field). The DH “activating function” is proportional notto the second-order derivative, but to the first-order derivative of thevoltage along the fiber axis. Accordingly, distance from the electricalfield locus affects the DH “activating function” less than it affectsthe DC “activating function.”

While fibers in the DC run in an axial direction, the neuronal elementsin the dorsal horn are oriented in many directions, includingperpendicular to the longitudinal axis of the spinal cord. However, asillustrated in FIG. 2, the dorsal horn stimulation technique describedin U.S. Provisional Patent Application Ser. No. 61/911,728, generates anelectrical field that is uniformly in one direction. There, thus,remains a need for an improved technique to stimulate the neuronalelements of the dorsal horn.

SUMMARY OF THE INVENTION

In accordance with the present inventions, a method of providing therapyto a patient having a medical condition (e.g., chronic pain) isprovided. The method comprises delivering electrical stimulation energy(e.g., anodic) to the spinal cord of the patient in accordance with astimulation program that preferentially stimulates dorsal horn neuronalelements over dorsal column neuronal elements in the spinal cord. In onemethod, the electrical stimulation energy is delivered to the spinalcord of the patient without creating the sensation of paresthesia in thepatient. The delivered electrical stimulation energy generates aplurality of electrical fields having different orientations thatstimulate the dorsal horn neuronal elements. For example, the pluralityof electrical fields may be orientated in different medio-lateraldirections or different rostro-caudal directions.

In one method, the electrical stimulation energy is delivered to thespinal cord of the patient as a pulsed electrical waveform, in whichcase, the plurality of electrical fields may be respectively generatedon a pulse-by-pulse basis. In another method, the plurality ofelectrical fields achieve temporal summation of stimulation in thedorsal horn neuronal elements. In still another method, the electricalstimulation energy is delivered from an electrical stimulation leadimplanted along a longitudinal axis of the spinal cord of the patient.The electrical stimulation lead may carry a plurality of electrodes, inwhich case, all of the electrodes may be activated to generate eachelectrical field.

An optional method further comprises cycling through the electricalfields multiple times. The electrical fields may, e.g., be generated thesame number of times for each electrical field cycle, generated adifferent number of times for each electrical field cycle, generated inthe same order during the electrical field cycles, generated in adifferent order during the electrical field cycles, or bursted on andoff at a burst frequency. In the latter case, the burst frequency maymatch a pathological burst frequency of medical condition.

In one method, the electrical stimulation energy may be delivered from aplurality of electrodes implanted adjacent the spinal cord of thepatient. In this case, the electrodes may be radially segmentedelectrodes. This method may further comprise determining a stimulationthreshold for each of the electrodes, and generating each of theelectrical fields based on the stimulation thresholds of the electrodes.In this case, determining the stimulation threshold for each of theelectrodes may comprise automatically delivering electrical energy fromeach of the electrodes at different amplitudes, automatically measuringan evoked compound action potential in response to the deliverance ofthe electrical energy from each of the electrodes, and automaticallyrecording the amplitude at which the evoked compound action potential ismeasured for each of the electrodes. Or, determining the stimulationthreshold for each of the electrodes may comprise automaticallydelivering electrical energy from each of the electrodes at differentamplitudes, acquiring feedback from the patient in response to thedeliverance of the electrical energy from each of the electrodes, andautomatically recording the amplitude at which paresthesia is perceivedby the patient for each of the electrodes.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a perspective view of a spinal cord, wherein the neuronalelements of the dorsal horn are particularly shown;

FIG. 2 is plan view of one embodiment of a SCS system arranged inaccordance with the present inventions;

FIG. 3 is a plan view of the SCS system of FIG. 2 in use to performspinal cord stimulation (SCS) on a patient;

FIG. 3 is a plan view of the SCS system of FIG. 1 in use to perform deepbrain stimulation (DBS) on a patient;

FIG. 4 is a plan view of an implantable pulse generator (IPG) and twoneurostimulation leads used in the SCS system of FIG. 1;

FIG. 5 is a cross-sectional view of one of the neurostimulation leads ofFIG. 4, taken along the line 5-5;

FIG. 6 is a perspective view of the spinal cord of a patient, whereinthe SCS system of FIG. 2 is used to generate multiple electrical fieldsthat stimulate the neuronal elements of the dorsal horn of the spinalcord;

FIGS. 7a-7c are plan views of one of the neurostimulation leads of FIG.4, particularly showing the generation of electrical fields at differentmedio-lateral directions;

FIGS. 8a-8c are plan views of one of the neurostimulation leads of FIG.4, particularly showing the generation of electrical fields at differentrostro-caudal directions; and

FIG. 9 is a timing diagram of a pulse pattern having electrical fieldsthat are generated on a pulse-by-pulse basis using the SCS system ofFIG. 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Turning first to FIG. 2, an exemplary SCS system 10 constructed inaccordance with the present inventions will now be described. The SCSsystem 10 generally comprises a plurality of neurostimulation leads 12(in this case, two percutaneous leads 12 a and 12 b), an implantablepulse generator (IPG) 14, an external remote control (RC) 16, a User'sProgrammer (CP) 18, an External Trial Stimulator (ETS) 20, and anexternal charger 22.

The IPG 14 is physically connected via two lead extensions 24 to theneurostimulation leads 12, which carry a plurality of electrodes 26arranged in an array. In the illustrated embodiment, theneurostimulation leads 12 are percutaneous leads, and to this end, theelectrodes 26 are arranged in-line along the neurostimulation leads 12.The number of neurostimulation leads 12 illustrated is two, although anysuitable number of neurostimulation leads 12 can be provided, includingonly one. Alternatively, a surgical paddle lead can be used in place ofone or more of the percutaneous leads. As will also be described infurther detail below, the IPG 14 includes pulse generation circuitrythat delivers electrical stimulation energy in the form of a pulsedelectrical waveform (i.e., a temporal series of electrical pulses) tothe electrode array 26 in accordance with a set of stimulationparameters. The IPG 14 and neurostimulation leads 12 can be provided asan implantable neurostimulation kit, along with, e.g., a hollow needle,a stylet, a tunneling tool, and a tunneling straw. Further detailsdiscussing implantable kits are disclosed in U.S. Application Ser. No.61/030,506, entitled “Temporary Neurostimulation Lead IdentificationDevice,” which is expressly incorporated herein by reference.

The ETS 20 may also be physically connected via percutaneous leadextensions 28 or external cable 30 to the neurostimulation lead 12. TheETS 20, which has similar pulse generation circuitry as the IPG 14, alsodelivers electrical stimulation energy in the form of a pulsedelectrical waveform to the electrode array 26 in accordance with a setof stimulation parameters. The major difference between the ETS 20 andthe IPG 14 is that the ETS 20 is a non-implantable device that is usedon a trial basis after the neurostimulation lead 12 has been implantedand prior to implantation of the IPG 14, to test the responsiveness ofthe stimulation that is to be provided. Thus, any functions describedherein with respect to the IPG 14 can likewise be performed with respectto the ETS 20.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation leads 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation programs after implantation. Oncethe IPG 14 has been programmed, and its power source has been charged orotherwise replenished, the IPG 14 may function as programmed without theRC 16 being present.

The CP 18 provides user detailed stimulation parameters for programmingthe IPG 14 and ETS 20 in the operating room and in follow-up sessions.The CP 18 may perform this function by indirectly communicating with theIPG 14 or ETS 20, through the RC 16, via an IR communications link 36.Alternatively, the CP 18 may directly communicate with the IPG 14 or ETS20 via an RF communications link (not shown).

The external charger 22 is a portable device used to transcutaneouslycharge the IPG 14 via an inductive link 38. Once the IPG 14 has beenprogrammed, and its power source has been charged by the externalcharger 22 or otherwise replenished, the IPG 14 may function asprogrammed without the RC 16 or CP 18 being present.

For the purposes of this specification, the terms “neurostimulator,”“stimulator,” “neurostimulation,” and “stimulation” generally refer tothe delivery of electrical energy that affects the neuronal activity ofneural tissue, which may be excitatory or inhibitory; for example byinitiating an action potential, inhibiting or blocking the propagationof action potentials, affecting changes inneurotransmitter/neuromodulator release or uptake, and inducing changesin neuro-plasticity or neurogenesis of tissue. For purposes of brevity,the details of the RC 16, ETS 20, and external charger 22 will not bedescribed herein. Details of exemplary embodiments of these componentsare disclosed in U.S. Pat. No. 6,895,280, which is expresslyincorporated herein by reference.

Referring to FIG. 3, the neurostimulation leads 12 are implanted at aninitial position within the spinal column 42 of a patient 40. Thepreferred placement of the neurostimulation leads 12 is adjacent, i.e.,resting near, or upon the dura, adjacent to the spinal cord area to bestimulated. In the illustrated embodiment, the neurostimulation leads 12are implanted along a longitudinal axis of the spinal cord of thepatient 40. Due to the lack of space near the location where theneurostimulation leads 12 exit the spinal column 42, the IPG 14 isgenerally implanted in a surgically-made pocket either in the abdomen orabove the buttocks. The IPG 14 may, of course, also be implanted inother locations of the patient's body. The lead extensions 24 facilitatelocating the IPG 14 away from the exit point of the neurostimulationleads 12. As there shown, the CP 18 communicates with the IPG 14 via theRC 16. After implantation, the IPG 14 can be operated to generate avolume of activation relative to the target tissue to be treated,thereby providing the therapeutic stimulation under control of thepatient.

Referring now to FIG. 4, the external features of the neurostimulationleads 12 a, 12 b and the IPG 14 will be briefly described. Theelectrodes 26 take the form of segmented electrodes that arecircumferentially and axially disposed about each of the respectiveneurostimulation leads 12 a, 12 b. By way of non-limiting example, andwith further reference to FIG. 5, each neurostimulation lead 12 maycarry sixteen electrodes, arranged as four rings of electrodes (thefirst ring consisting of electrodes E1-E4; the second ring consisting ofelectrodes E5-E8; the third ring consisting of electrodes E9-E12; andthe fourth ring consisting of electrodes E13-E16) or four axial columnsof electrodes (the first column consisting of electrodes E1, E5, E9, andE13; the second column consisting of electrodes E2, E6, E10, and E14;the third column consisting of electrodes E3, E7, E11, and E15; and thefourth column consisting of electrodes E4, E8, E12, and E16). The actualnumber and shape of leads and electrodes will, of course, vary accordingto the intended application. Further details describing the constructionand method of manufacturing percutaneous stimulation leads are disclosedin U.S. patent application Ser. No. 11/689,918, entitled “Lead Assemblyand Method of Making Same,” and U.S. patent application Ser. No.11/565,547, entitled “Cylindrical Multi-Contact Electrode Lead forNeural Stimulation and Method of Making Same,” the disclosures of whichare expressly incorporated herein by reference.

The IPG 14 comprises an outer case 50 for housing the electronic andother components (described in further detail below). The outer case 50is composed of an electrically conductive, biocompatible material, suchas titanium, and forms a hermetically sealed compartment wherein theinternal electronics are protected from the body tissue and fluids. Insome cases, the outer case 50 may serve as an electrode. The IPG 14further comprises a connector 52 to which the proximal ends of theneurostimulation leads 12 mate in a manner that electrically couples theelectrodes 26 to the internal electronics (described in further detailbelow) within the outer case 50. To this end, the connector 52 includestwo ports (not shown) for receiving the proximal ends of the leads 12.In the case where the lead extensions 24 are used, the ports may insteadreceive the proximal ends of such lead extensions 24.

As briefly discussed above, the IPG 14 includes circuitry that provideselectrical stimulation energy to the electrodes 26 in accordance with aset of parameters. Such stimulation parameters may comprise electrodecombinations, which define the electrodes that are activated as anodes(positive), cathodes (negative), and turned off (zero), percentage ofstimulation energy assigned to each electrode (fractionalized electrodeconfigurations), and electrical pulse parameters, which define the pulseamplitude (measured in milliamps or volts depending on whether the IPG14 supplies constant current or constant voltage to the electrode array26), pulse width (measured in microseconds), pulse rate (measured inpulses per second), and burst rate (measured as the stimulation onduration X and stimulation off duration Y). As will be described infurther detail below, the IPG 14 also includes circuitry that provideselectrical signals, and measured electrical impedance in response to theelectrical signals.

With respect to the pulsed electrical waveform provided during operationof the SCS system 10, electrodes that are selected to transmit orreceive electrical energy are referred to herein as “activated,” whileelectrodes that are not selected to transmit or receive electricalenergy are referred to herein as “non-activated.” Electrical energydelivery will occur between two (or more) electrodes, one of which maybe the IPG case 50, so that the electrical current has a path from theenergy source contained within the IPG case 50 to the tissue and a sinkpath from the tissue to the energy source contained within the case.Electrical energy may be transmitted to the tissue in a monopolar ormultipolar (e.g., bipolar, tripolar, etc.) fashion.

Monopolar delivery occurs when a selected one or more of the leadelectrodes 26 is activated along with the case 50 of the IPG 14, so thatelectrical energy is transmitted between the selected electrode 26 andcase 50. Monopolar delivery may also occur when one or more of the leadelectrodes 26 are activated along with a large group of lead electrodeslocated remotely from the one or more lead electrodes 26 so as to createa monopolar effect; that is, electrical energy is delivered from the oneor more lead electrodes 26 in a relatively isotropic manner. Bipolardelivery occurs when two of the lead electrodes 26 are activated asanode and cathode, so that electrical energy is transmitted between theselected electrodes 26. Tripolar delivery occurs when three of the leadelectrodes 26 are activated, two as anodes and the remaining one as acathode, or two as cathodes and the remaining one as an anode.

The IPG 14 comprises electronic components, such as a memory 54,controller/processor (e.g., a microcontroller) 56, monitoring circuitry58, telemetry circuitry 60, a battery 62, stimulation output circuitry64, and other suitable components known to those skilled in the art.

The memory 54 is configured for storing programming packages,stimulation parameters, measured physiological information, and otherimportant information necessary for proper functioning of the IPG 14.The microcontroller 56 executes a suitable program stored in memory 54for directing and controlling the neurostimulation performed by IPG 14.The monitoring circuitry 58 is configured for monitoring the status ofvarious nodes or other points throughout the IPG 14, e.g., power supplyvoltages, temperature, battery voltage, and the like. Notably, theelectrodes 26 fit snugly within the patient, and because the tissue isconductive, electrical measurements can be taken between the electrodes26. Thus, the monitoring circuitry 58 is configured for taking suchelectrical measurements (e.g., electrode impedance, field potential,evoked action potentials, etc.) for performing such functions asdetecting fault conditions between the electrodes 26 and the stimulationoutput circuitry 64, determining the coupling efficiency between theelectrodes 26 and the tissue, determining the posture/patient activityof the patient, facilitating lead migration detection.

More significant to the present inventions, an evoked potentialmeasurement technique can be used to calibrate the stimulation energydelivered to the spinal cord. The evoked potential measurement techniquemay be performed by generating an electrical field at one of theelectrodes 26, which is strong enough to depolarize the neurons adjacentthe stimulating electrode beyond a threshold level, thereby inducing thefiring of action potentials (APs) that propagate along the neuralfibers. Such stimulation is preferably supra-threshold, but notuncomfortable. A suitable stimulation pulse for this purpose is, forexample, 4 mA for 200 μs. While a selected one of the electrodes 26 isactivated to generate the electrical field, a selected one or ones ofthe electrodes 26 (different from the activated electrode) is operatedto record a measurable deviation in the voltage caused by the evokedpotential due to the stimulation pulse at the stimulating electrode.

The telemetry circuitry 60, including an antenna (not shown), isconfigured for receiving programming data (e.g., the operating programand/or stimulation parameters, including pulse patterns) from the RC 16and/or CP 18 in an appropriate modulated carrier signal, which theprogramming data is then stored in the memory 54. The telemetrycircuitry 60 is also configured for transmitting status data to the RC16 and/or CP 18 in an appropriate modulated carrier signal. The battery62, which may be a rechargeable lithium-ion or lithium-ion polymerbattery, provides operating power to IPG 14. The stimulation outputcircuitry 64 is configured for, under control of the microcontroller 56,generating and delivering electrical energy, in the form of electricalpulse trains, to each of the electrodes 26, as well as any electricalsignals needed for acquiring electrical measurements.

Notably, while the microcontroller 56 is shown in FIG. 4 as a singledevice, the processing functions and controlling functions can beperformed by a separate controller and processor. Thus, it can beappreciated that the controlling functions performed by the IPG 14 canbe performed by a controller, and the processing functions performed bythe IPG 14 can be performed by a processor. Additional detailsconcerning the above-described and other IPGs may be found in U.S. Pat.No. 6,516,227, U.S. Patent Publication No. 2003/0139781, and U.S. patentapplication Ser. No. 11/138,632, entitled “Low Power Loss CurrentDigital-to-Analog Converter Used in an Implantable Pulse Generator,”which are expressly incorporated herein by reference. It should be notedthat rather than an IPG, the SCS system 10 may alternatively utilize animplantable receiver-modulator (not shown) connected to the leads 12. Inthis case, the power source, e.g., a battery, for powering the implantedreceiver, as well as control circuitry to command thereceiver-modulator, will be contained in an external controllerinductively coupled to the receiver-modulator via an electromagneticlink. Data/power signals are transcutaneously coupled from acable-connected transmission coil placed over the implantedreceiver-modulator. The implanted receiver-modulator receives the signaland generates the stimulation in accordance with the control signals.

More significant to the present inventions, the SCS system 10 deliverselectrical stimulation energy to the spinal cord of the patient inaccordance with a stimulation program that preferentially stimulatesdorsal horn neuronal elements over dorsal column neuronal elements inthe spinal cord.

To this end, the current delivered from the electrodes 26 isfractionalized, such that the electrical field generated by theneurostimulation lead(s) 12 has an electrical field strength in thelongitudinal direction that is approximately equal, resulting in avoltage gradient of approximately zero along the dorsal column. Thissubstantially constant electrical field forms a small longitudinalgradient, which minimizes activation of the large myelinated axons inthe dorsal column. In contrast, the electrical field generated by theneurostimulation lead(s) 12 has an electrical field strength in thetransverse direction that substantially differs, resulting a strongvoltage gradient in the dorsal horn. In particular, the transverseelectrical field strength is greatest adjacent the neurostimulationlead(s) 12 and falls off laterally, resulting in a sizable gradient inthe transverse direction, which activates the neural cell terminals inthe dorsal horn. Thus, the substantially constant longitudinalelectrical field and the large gradient in the transverse electricalfield favor stimulation of dorsal horn neuronal elements over dorsalcolumn neuronal elements. This electrical field makes the dorsal columnneuronal elements even less excitable relative to the dorsal hornneuronal elements. In this manner, the perception of paresthesia iseliminated or at least minimized. In the illustrated embodiment, the allof the electrodes 26 on the neurostimulation leads 12 are preferablyactivated to maximize the stimulation of the dorsal horn neuronalelements along the leads 12.

Calibration techniques (described below) may be used to determine theproper current fractionalization for the electrodes 26. With the currentfractionalized to a plurality of electrodes 26 on the neurostimulationlead 12, the resulting field can be calculated by superimposing thefields generated by the current delivered to each electrode 26. In theillustrated embodiment, the electrodes 26 on the neurostimulation lead12(s) are anodic, while the outer case 44 of the IPG 14 is cathodic. Inthis manner, a monopolar anodic electrical field is generated by the SCSsystem 10. Further details discussing techniques for preferentiallystimulating dorsal horn neuronal elements over dorsal column neuronalelements are described in U.S. Provisional Patent Application Ser. No.61/911,728, entitled “Systems and Methods for Delivering Therapy to theDorsal Horn of a Patient,” which is expressly incorporated herein byreference.

Significantly, the SCS system 10 delivers the electrical energy to thespinal cord of the patient by generating a plurality of electricalfields having different orientations that target the differentdirections/orientations of the dorsal horn neuronal elements, asillustrated in FIG. 6. In this manner, all, or at least a significantamount of, the dorsal horn neuronal elements will be stimulated by atleast one of the electrical fields.

In the illustrated embodiment, the electrical fields are oriented indifferent medio-lateral directions (i.e., the direction of theelectrical fields as projected on a transverse plane through the spinalcord). To generate electrical fields in different medio-lateraldirections, the electrodes 26 may have different currentfractionalizations in the radial direction. For example, referring backto FIG. 5, the first column of electrodes E1, E5, E9, and E13 maydeliver 50% of the anodic current, and the second column of electrodesE2, E6, E10, and E14 may deliver the remaining 50% of the anodic currentto orient the electrical field in one medio-lateral direction, asillustrated in FIG. 7a . The first column of electrodes El, E5, E9, andE13 may deliver 75% of the anodic current, and the second column ofelectrodes E2, E6, E10, and E14 may deliver the remaining 25% of theanodic current to orient the electrical field in one medio-lateraldirection to orient the electrical field in another medio-lateraldirection, as illustrated in FIG. 7b . The first column of electrodesE1, E5, E9, and E13 may deliver 100% of the anodic current to orient theelectrical field in one medio-lateral direction to orient the electricalfield in another medio-lateral direction, as illustrated in FIG. 7 c.

Although it is desirable that the electrical fields preferentiallystimulate dorsal horn neuronal elements over the dorsal column neuronalelements, as discussed above, the electrical fields may still beoriented in different rostro-caudal directions (i.e., the direction ofthe electrical fields as projected on a longitudinal plane through thespinal cord), although preferably not in an orientation that will resultin the perception of paresthesia. To generate electrical fields indifferent rostro-caudal directions, the electrodes 26 may have differentcurrent fractionalizations in the longitudinal direction. For example,referring back to FIG. 5, each of the first ring of electrodes E1-E4,the second ring of electrodes E5-E8; the third ring of electrodesE9-E12, and the fourth ring of electrodes E13-E16 may deliver 25% of theanodic current to orient the electrical field in one rostro-caudaldirection, as illustrated in FIG. 8a . The first ring of electrodesE1-E4, may deliver 10% of the anodic current, the second ring ofelectrodes E5-E8 may deliver 25% of the anodic current, the third ringof electrodes E9-E12 may deliver 30% of the anodic current, and thefourth ring of electrodes E13-E16 may deliver 35% of the anodic currentto orient the electrical field in one rostro-caudal direction, asillustrated in FIG. 8b . The first ring of electrodes E1-E4, may deliver5% of the anodic current, the second ring of electrodes E5-E8 maydeliver 20% of the anodic current, the third ring of electrodes E9-E12may deliver 35% of the anodic current, and the fourth ring of electrodesE13-E16 may deliver 40% of the anodic current to orient the electricalfield in one rostro-caudal direction, as illustrated in FIG. 8 c.

The different electrical fields generated by the SCS system 10preferably achieve a temporal summation of stimulation in the dorsalhorn neuronal elements. To ensure this temporal summation ofstimulation, the electrical fields can be generated respectively on apulse-by-pulse basis. For example, as illustrated in FIG. 9, a firstelectrical field can be generated by the electrodes 26 (using a firstcurrent fractionalization) during a first electrical pulse of the pulsedwaveform, a second different electrical field can be generated by theelectrodes 26 (using a second different current fractionalization)during a second electrical pulse of the pulsed waveform, a thirddifferent electrical field can be generated by the electrodes 26 (usinga third different current fractionalization) during a third electricalpulse of the pulsed waveform, a fourth different electrical field can begenerated by the electrodes 26 (using a fourth different currentfractionalized) during a fourth electrical pulse of the pulsed waveform,and so forth. Further details on the delivery of different electricalfields on a pulse-by-pulse basis are set forth in U.S. ProvisionalPatent Application Ser. No. 62/020,836, which is expressly incorporatedherein by reference.

The electrical fields generated by the SCS system 10 may be rotated orcycled through multiple times under a timing scheme. The electricalfield cycling can be accomplished in any one of a variety of manners. Inone embodiment, the different electrical fields are generated in thesame (or regular) order during the electrical field cycles. For example,if four electrical fields labeled 1-4 are generated, the order in whichthese electrical fields are generated may be {2, 3, 1, 4}, {2, 3, 1, 4},{2, 3, 1, 4}, etc. The different electrical field may alternatively begenerated in a different order (or irregular) during the electricalfield cycles. For example, the order in which electrical fields 1-4 aregenerated may be {1, 2, 3, 4}, {3, 1, 2, 4}, {4, 1, 3, 2}, {1, 2, 3, 4},etc.

Although electrical fields 1-4 have been described as being generatedthe same number of times for each electrical field cycle (in the casesabove, one time per each cycle), the electrical fields 1-4 may begenerated a different number of times for each electrical field cycle.That is, the cycling can be biased towards one electrical field relativeto another electrical field. For example, electrical fields 1-4 may begenerated during the electrical field cycles as follows: {1, 2, 2, 2, 3,3, 4}, {1, 2, 2, 2, 3, 3, 4}, {1, 2, 2, 2, 3, 3, 4}, etc. Thus, in thiscase, electrical field 1 is generated once, electrical field 2 isgenerated thrice, electrical field 3 is generated twice, and electricalfield 4 is generated once per electrical field cycle.

In the above exemplary cases, the electrical fields 1-4 can be generatedat a continuous pulse rate. However, in an optional embodiment, theelectrical field cycles can be bursted on and off. For example, anelectrical field cycle {2, 3, 1, 4} can be repeatedly bursted at adefined frequency (e.g., a cycle burst every 100 ms). In oneparticularly useful embodiment, the burst frequency matches thepathological burst frequency of the neurological signals that cause thechronic pain.

Although the interpulse interval (i.e., the time between adjacentpulses), pulse amplitude, and pulse duration during the electrical fieldcycles has been described as being uniform, the interpulse interval,pulse amplitude, and/or pulse duration may vary within the electricalfield cycle, as described in U.S. Provisional Patent Application Ser.No. 62/020,836, which has previously been incorporated herein byreference.

Because the stimulation threshold (i.e., the electrical current neededon an activated electrode to stimulate adjacent tissue) varies frompatient to patient and from electrode 26 to electrode 26 within apatient, a more accurate fractionalization of the current betweenelectrodes 26 to generate the various electrical fields requiresmodification of the fractionalization based on the stimulation thresholdat each electrode. To this end, the electrodes may be calibrated bydetermining the stimulation threshold level (i.e., the electricalcurrent needed on an activated electrode to stimulate adjacent tissue)for each of the electrodes and using the stimulation threshold levels todetermine the fractionalized electrical current values for generatingthe electrical fields. This calibration technique may involvecalculating a driving force directed to each electrode.

Preferably, the stimulation threshold for each of the electrodes 26 isdetermined by automatically delivering electrical energy from each ofthe electrodes 26 at different amplitudes, automatically measuring anevoked compound action potential in response to the deliverance of theelectrical energy from each of the electrodes 26, and automaticallyrecording the amplitude at which the evoked compound action potential ismeasured for each of the electrodes 26. The electrical energy may bedelivered to each electrode in a monopolar mode as either anodic orcathodic electrical energy. This automated electrode calibrationtechnique can be updated periodically or in response to a particularevent, such as a posture change of the patient.

Determination of the stimulation thresholds may be binary in nature,meaning that the presence or absence of a measured evoked compoundaction potential either indicates that a stimulation threshold has beenreached or not reached for a particular electrode, or the determinationof the stimulation thresholds may be more sophisticated in nature. Themaximum amplitude of the electrical energy delivered from each electrodeshould be managed so that the patient does not perceive the electricalstimulation too much, although since single electrical pulses can beused, the patient may not perceive much even at amplitudes that would bestrong enough to cause continuous stimulation.

Optionally, the stimulation threshold determined previous electrodes,including the first calibrated electrode, may be used as a startingpoint for the stimulation threshold determination for subsequentelectrodes, so that the amplitude need not be initially set to zero foreach subsequently electrode in order to speed up the calibrationprocess. For example, the electrical energy may be transitioned betweenelectrodes at an amplitude where the patient barely perceivesstimulation, a comfortable level, or some other constant level.

Alternatively, the stimulation threshold for each of the electrodes 26may be determined by automatically delivering electrical energy fromeach of the electrodes 26 at different amplitudes, acquiring feedbackfrom the patient, and in particular communicating when the patientperceives paresthesia, in response to the deliverance of the electricalenergy from each of the electrodes 26, and automatically recording theamplitude at which paresthesia is perceived by the patient for each ofthe electrodes 26. However, it should be appreciated that measuringevoked compound action potentials, as opposed to relying on subjectivepatient feedback, is objective in nature, can be performed quickly, andcan be determined using a relatively small number of electrical pulsesas opposed perceiving paresthesia, which requires a relatively largenumber of electrical pulses.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. A non-transitory machine-readable mediumincluding instructions, which when executed by a machine, cause themachine to provide therapy using implanted electrodes to a patienthaving a medical condition, including deliver electrical stimulationenergy as a pulsed electrical waveform to a spinal cord of the patientin accordance with a stimulation program that preferentially stimulatesdorsal horn neuronal elements over dorsal column neuronal elements inthe spinal cord, including generate a first electrical field having afirst orientation during at least a first pulse in the pulsed electricalwaveform, generate a second electrical field having a second orientationduring at least a second pulse in the pulsed electrical waveform, andgenerate a third electrical field having a third orientation during atleast a third pulse in the pulsed electrical waveform the first, secondand third orientations being different from each other.
 2. Thenon-transitory machine-readable medium of claim 1, further comprisinginstructions, which when executed by the machine, cause the machine togenerate the electrical stimulation energy in repeating electrical fieldcycles, wherein each of the repeating electrical field cycles includesthe first electrical field, the second electrical field, and the thirdelectrical field.
 3. The non-transitory machine-readable medium of claim2, wherein the electrical fields are generated a same number of pulsesin each electrical field cycle.
 4. The non-transitory machine-readablemedium of claim 2, wherein the electrical fields are generated adifferent number of pulses in each electrical field cycle.
 5. Thenon-transitory machine-readable medium of claim 2, wherein theelectrical fields are generated in a same order during the electricalfield cycles.
 6. The non-transitory machine-readable medium of claim 2,wherein the electrical fields are generated in a different order duringthe electrical field cycles.
 7. The non-transitory machine-readablemedium of claim 2, wherein the repeating electrical field cycles arebursted on and off at a burst frequency.
 8. The non-transitorymachine-readable medium of claim 2, wherein the electrical field cyclesinclude a bias toward one of the first, second and third electricalfields relative to the other ones of the first, second and thirdelectrical fields.
 9. The non-transitory machine-readable medium ofclaim 1, wherein the first, second and third electrical fields arerotated through under a timing scheme.
 10. The non-transitorymachine-readable medium of claim 1, wherein the instructions, which whenexecuted by the machine, cause the machine to deliver the electricalstimulation energy as the pulsed electrical waveform to the spinal cordinclude instructions, which when executed by the machine, cause themachine to provide an electrical field with a longitudinal voltagegradient along the spinal cord that is smaller than a voltage gradientin a transverse direction.
 11. The non-transitory machine-readablemedium of claim 10, wherein the first, second and third orientations aredifferent medio-lateral directions.
 12. The non-transitorymachine-readable medium of claim 1, wherein the first electrical fieldis generated using a first current fractionalization, the secondelectrical field is generated using a second current fractionalization,and the third electrical field is generated using a third currentfractionalization.
 13. A system for providing a therapy to a patientusing electrodes, comprising: a controller/processor configured tocooperate with stimulation output circuitry to provide the therapy tothe patient in accordance with a stimulation program that preferentiallystimulates dorsal horn neuronal elements over dorsal column neuronalelements in the spinal cord including deliver electrical stimulationenergy as a pulsed electrical waveform to a spinal cord of the patient,including generate a first electrical field having a first orientationduring at least a first pulse in the pulsed electrical waveform, asecond electrical field having a second orientation during at least asecond pulse in the pulsed electrical waveform, and a third electricalfield having a third orientation during at least a third pulse in thepulsed electrical waveform, the first, second and third orientationsbeing different from each other.
 14. The system of claim 13, wherein thecontroller/processor is configured to cooperate with the stimulationoutput circuitry to deliver the electrical stimulation energy inrepeating electrical field cycles, wherein each of the repeatingelectrical field cycles includes the first electrical field, the secondelectrical field, and the third electrical field.
 15. The system ofclaim 14, wherein the controller/processor is configured to cooperatewith the stimulation output circuitry to burst the repeating electricalfield cycles on and off at a burst frequency.
 16. The system of claim14, wherein the controller/processor is configured to cooperate with thestimulation output circuitry to bias one of the first, second and thirdelectrical fields relative to the other ones of the first, second andthird electrical fields.
 17. The system of claim 13, wherein thecontroller/processor is configured to cooperate with the stimulationoutput circuitry to use a timing scheme to rotate through the first,second and third electrical fields.
 18. The system of claim 13, whereinthe controller/processor is configured to cooperate with the stimulationoutput circuitry to provide an electrical field with a longitudinalvoltage gradient along the spinal cord that is smaller than a voltagegradient in a transverse direction.
 19. The system of claim 18, whereinthe first, second and third orientations are different medio-lateraldirections.
 20. The system of claim 13, wherein the controller/processoris configured to cooperate with the stimulation output circuitry togenerate the first electrical field using a first currentfractionalization, the second electrical field using a second currentfractionalization, and the third electrical field using a third currentfractionalization.